In a conventional ophthalmoscope, the light illuminating the eyepiece or image detector is a combination of light from the object being viewed and light scattered from the region surrounding the object. The resultant image therefore includes a sharply focused image from the object and a superposition of out-of-focus images caused by scattered light from the region surrounding the object. This light originates in planes axially displaced from the plane containing the object being viewed (hereinafter referred to as the "object plane") and from points on the object plane but laterally displaced from the object being viewed. The reduction in contrast caused by these out-of-focus images can seriously interfere with the use of the ophthalmoscope to observe the retina.
A confocal ophthalmoscope rejects light originating from the region surrounding the object being viewed. It does so by directing light from the object through a pinhole located confocally with the object and with the source. As a result, light rays originating from the object pass through the pinhole to an image detector or other output element. In contrast, a major portion of the light rays originating both from planes other than the object plane and from points on the object plane but laterally displaced from the object do not pass through the pinhole. This permits the observation of objects deep within a three-dimensional structure, for example the retina of an eye, without interference from objects lying in planes above or below the object plane.
Because a confocal ophthalmoscope, as described above, observes an image through a pinhole, its field of view is severely limited. Such an ophthalmoscope can observe no more than a single point at a time. To observe additional points on an object, the pinhole's image can be scanned across the object plane. Conventional methods for scanning a pinhole light source are described in U.S. Pat. No. 4,213,678 and in U.S. Pat. No. 4,765,730. These methods typically use a rotating polygonal mirror or other mechanical apparatus to scan a laser beam across the pupil. Mechanical methods such as these are limited in performance by their mechanical inertia.
Additionally, the pinhole is typically constrained to be simply connected, e.g. circular, as opposed, for example, to an annulus or a more complex shape. An annular aperture is of particular interest in ophthalmologic applications. By using an annular illuminating source instead of a circular one, an ophthalmologist can exclude returning light originating from the point in the object confocal to the source and collect multiply scattered light originating from other points. Thus, by controlling the diameter of the annulus, the ophthalmologist can collect light multiply scattered from other parts of the retina. Although the image formed by collecting light in this manner mayl not be sharply focused, it will be a relatively bright image because the cross sectional area of the annular aperture is typically larger than the cross sectional area of the pinhole.
Further description of conventional practice is set forth in the literature, including Confocal Optical Microscopy, by R. H. Webb, Rep. Prog. Phys. 59 (1996) 427-471.
In light of the foregoing, there is a need for confocal ophthalmoscopes in which one or more light sources of specified size and shape can be programmed or otherwise reliably sequenced to illuminate an object plane within the eye or to traverse a specified path across an object plane within the eye and which can, with a minimum of light loss, transmit the corresponding image from the object plane within the eye to an image detector or other output element.